After upgrading our app from Ruby 3.1.2 to 3.2.2, Rails 6.1.4 to 7.0 and mongoid 7.5 to 8.0.3 when using embeds_many and embeds_one with the store_as attribute the app is failing with this message when trying to assign attributes undefined method `nested_builder' for nil:NilClass. 

After tracing this error through to the mongoid gem, I could see that it came from this line, and seems to happen because the key for embedded associations with a store_as defined is different in the pending_relations hash to what it is in the relations hash. I can fix this issue with a work-around, changing the code in the `pending_attribute?` method to:

def pending_attribute?(key, value)
  name = key.to_s

  aliased = if aliased_associations.key?(name)
              aliased_associations[name]
            else
              name
            end

  if relations.has_key?(aliased)
    pending_relations[aliased.to_s] = value
    return true
  end
  if nested_attributes.has_key?(aliased)
    pending_nested[name] = value
    return true
  end
  return false
end

Fixes my specific problem (storing the value in the ray with the key of `aliased.to_s` rather than the key of `name` that it was before.

But I don't think this gets to the root cause as this code was working fine before, and did not change when upgrading from mongoid 7.5 to 8.03